The present invention relates to an antenna which communicates an electromagnetic wave, and more particularly, to an antenna which can be used for waves ranging from an MF (medium frequency) band to a VHF (very high frequency) band and a UHF (ultra high frequency) band.
Related antennas can be roughly classified into the following five categories, according to operating principle.
A first type of antenna is one which produces a voltage as a result of an electric field acting on a conductor of linear shape or an analogous shape. A second type of antenna is one which produces a voltage across the ends of an annular conductor from an electromagnetic wave penetrating therethrough. A third type of antenna is one which converges an electromagnetic wave into an opening in a conductor by utilizing an eddy current developing around the opening. A fourth type of antenna is one which converges magnetic flux by a high-frequency magnetic substance and converts the magnetic flux into voltage by an electric coil. A fifth type of antenna is one which converges an electromagnetic wave by utilizing reflection developing in the surface of a parabolic conductor.
Specific names of these antennas as follows:
The first type of antenna includes an inverted L-shaped antenna used in a frequency band shorter than short wave, and a dipole antenna and a mono-pole antenna which are used for a high frequency band or higher. Further, the first type of antenna includes a Yagi antenna which is utilized for receiving an FM broadcast or a TV signal. The Yagi antenna is constituted by providing a dipole antenna with a wave director and a reflector.
The second type of antenna is called a loop antenna.
The third type of antenna is called a slot antenna. This slot antenna is employed by cell sites for a portable cellular phone or as a flat antenna for receiving satellite broadcast.
The fourth type of antenna is called a ferrite antenna or a bar antenna. A ferrite core is used as high frequency magnetic substance.
The fifth type of antenna is called a parabolic antenna. The parabolic antenna is used for communicating radio waves of higher frequency than VHF or is used as a radar antenna.
The maximum output voltage of each of the first and third antennas is defined as the product of field intensity and the length of an antenna. The first and third types of antennas possess the drawback of not being expected to be able to acquire a great antenna gain. In order to compensate for the drawback, a plurality of the third type of antennas are connected in parallel to acquire great output power at a load of low impedance.
The second type of antenna; that is, a loop antenna, is for detecting magnetic flux passing through a plane constituted of a coil. An output voltage of the loop antenna can be increased by increasing the size of a coil and the winding number thereof. However, when the winding number of a coil of great area is increased, the inductance of the coil and stray capacitance existing between lines of the coil are increased, thus reducing the resonance frequency of the coil. Since there is a necessity of selecting, as the resonance frequency, a frequency higher than a frequency to be used for communication, restrictions are imposed on the area of a coil and the winding number thereof.
The fourth type of antenna; that is, a ferrite antenna, enables reduction in the area of a coil by converging magnetic flux through use of a ferrite core. Since the winding number of a coil can be increased, the ferrite antenna has been widely adopted as a high-sensitivity MF antenna. At a frequency of higher than 1 MHz, permeability of ferrite magnetic material drops, in substantially inverse proportion to frequency. Since the highest operation frequency of magnetic material is about 10 GHz, the ferrite antenna possesses the drawback of not being able to be applied to frequencies of higher than the VHF range.
The fifth, parabolic antenna converges an electromagnetic wave through use of a parabolic reflection mirror, the outer dimension of the mirror being greater than the wavelength of a subject electromagnetic wave, thereby acquiring a high antenna gain. Since the antenna has high directionality, the antenna is used primarily for fixed stations.